The goal of this research is an understanding of how hormones, particularly the steroids - androgens and estrogens, control the sexually specific development of target neurons and muscle to produce the mature male or female phenotype. These questions are central to an understanding of male- and female-typical clinical disorders. Hormone-directed cell development underlies malignancies of prostate and breast, disorders of secondary sexual differentiation such as androgen-insensitivity and pseudohemaphodism and at least some aspects of the establishment of gender identity. This proposal is focussed on the control of cell type in laryngeal muscle fibers and at the laryngeal neuromuscular junction of Xenopus laevis. This experimental preparation provides an excellent model system in which to study the cellular and molecular bases of secondary sexual differentiation. The development of the vocal organ, or larynx, is highly sexually dimorphic as a result of sex differences in the endocrine milieu. The isolated larynx can produce actual vocalizations permitting identification of male- and female-specific electrophysiological and muscular characteristics which underlie the different vocal repertoires of adult males and females. These differences include a sex difference in synaptic efficacy - weak synapses in males versus strong synapses in female - and in muscle fiber type -rapidly contracting fibers in males and slowly contracting fibers in females. The control of synaptic strength and the ability of synapses to increase in efficacy with use will be examined in males and female larynges using electrophysiological and dye imaging methods. Calcium entry into the presynaptic terminal will be followed using calcium-sensitive, fluorescent dyes; mechanisms underlying sex differences - including sex differences in ion channels and in other aspects of transmitter release- will be examined. The developmental origins of sex differences in synaptic efficacy will be explored. Preliminary results strongly suggest that sex differences in synaptic strength are regulated by exposure to estrogen. The way in which estrogen acts on the laryngeal synapse to promote neurotransmitter release will be studied and cellular and molecular mechanisms underlying estrogen action determined using hormone ablation and restoration, immunocytochemistry and in situ hybridization. A laryngeal-specific, androgen-regulated myosin heavy chain gene (LM) has been identified as a strong molecular candidate for the regulation of fiber type in the larynx. The role played by LM in fiber type determination will be examined in vivo and in vitro using molecular and histological approaches. Preliminary results suggest that LM is an embryonic myosin heavy chain (MHC) isoform. The expression of LM and of other Xenopus MHC's will be examined in tissue extracts, whole mounts and sections using hybridization techniques. The of LM and of other MHC's in laryngeal muscle by androgen and thyroid hormone will be explored in vivo and in vitro using endocrine ablation and replacement. The lineage of myoblasts giving rise to laryngeal fibers will be examined and the role of LM-expressing myoblasts in fiber type switching examined.